The main sequence star refers to the main sequence starHerotuMain order bandOffixed star。Astronomically, the main sequence stars can show the evolution process of starsHerotuThe distribution from the upper left corner to the lower right corner is also called the stars on the main sequence belt.
The main sequence band is a continuous and unique stellar band, which is plotted by the relative luminosity of color. This color luminosity map is Eichener's·HerzpoonAnd Henry Norris·RussellThe famousHerotu。The stars in this main sequence belt are the so-called main sequence stars.[1-2]
After star formation, it is carried out in the core with high heat and densitynuclear fusionReaction, hydrogen atoms into helium, and produce energy.The position of the star at this stage in the main sequence belt is mainly due to its mass, chemical composition or other factors.
All the main sequence stars are inHydrostatic balanceState, the thermal pressure from the expansion of the hot core andGravitational collapseThe inward pressure is in balance.Core temperature and pressure have a strong correlation with productivity and help maintain balance.The energy generated by the core is transferred to the surface and radiated out through the photosphere.Energy is transferred by radiation or convection, and convection transfer will occur in this areatemperature gradient, higher opacity, or both.
At the beginning of the 20th century, more information about star types and distances became available.SiderealspectrumIt has proved to have unique functions and can be used for classification.HarvardAnne CannonAnd Edward·PickeringThe developed classification system will become well-known in the futureHarvard ClassificationSystem, published in Harvard Annals in 1901.[3]
Danish astronomer in Potsdam in 1906Ehina HerzpoonNote that the reddest stars - K and M in the Harvard system - can be divided into two different groups.These stars are either brighter or much dimmer than the sun.In order to distinguish these two groups, he called them“SuperStarAnddwarf star". The next year he began to study star clusters; a large number of stars at about the same distance belong to the same star group. He published the first picture of the color of these stars relative to brightness, which shows a highlighted and continuous series of stars, which he called“Main order band”。[4]
stayPrinceton University ，Henry Norris RussellSimilar studies have also been done as follows.He studies the spectral classification of stars and their corrected true brightness over distance - theirAbsolute magnitude。To achieve this, he used a series of stars with exact parallax in the Harvard classification system.When it drew the map of the spectrum of these stars corresponding to the absolute magnitude, he found that these dwarfs followed a clear relationship, which enabled him to predict the brightness of dwarfs reasonably and correctly[5]。
The red stars observed by Herzpoon,dwarf starIt also follows the spectral photometric relationship discovered by Russell.However, the giant star is still much brighter than the dwarf star, and does not follow the same relationship.Russell believed that "a giant star must have low density or large surface brightness, which is just the opposite of the fact of a dwarf star".The same curve also shows a few white dark stars[5]。
In 1933, Bent Strongen introduced Herot to show the relationship between brightness and spectral classification[6]。This name reflects that this method was developed in parallel by Herzpoon and Russell in the early 20th century.Like the stellar evolution model developed in the 1930s, it shows that stars have consistent chemical composition, and there is a correlation between the mass and radius of stars.That is, for a given star mass and composition, there is a unique star radius and photometric solution.This is called Russell Walker theorem, named after Henry Norris Russell and Heine Walker.Through this theorem, once the chemical composition of a star and its position in the main sequence band are known, the mass and radius of the star have been determined.However, it was later discovered that this theorem did not apply to stars with different compositions[7]。
W. W. Morgan and PC. Kennan published an improved star classification in 1943[8]。The Morgan Kennan classification (MK system) selects the spectrum of each star - based on the Harvard classification system - and photometric classification.The Harvard classification system marks each star with different letters according to its hydrogen line intensity before knowing the relationship between spectrum and temperature.After sorting by temperature and sifting out repeated classifications, the spectral types of stars follow the order of temperature from high to low and temperature from blue to red, and the sequence is OB、A、F、G、K、And M (The popular method used to remember the star classification sequence is Oh Be A Fine Girl/Guy, Kiss Me ").The brightness classification is based on the reduction of brightness from I to V,Main order bandOf the stars are classified as V.[9]
Formation and evolution
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Formation and evolution of main sequence stars
When a protostar collapses from a giant molecular cloud composed of gas and dust in the interstellar medium, the initial composition is uniform, with about 70% hydrogen and 28% helium in mass, as well as other traceable elements[10]。
The initial mass of a star is determined byMolecular cloudThe condition of the position of (the mass distribution of newly formed stars is based onInitial mass functionExperience to describe).When the collapse begins, the former main sequence star generates energy through gravitational contraction.When the proper density is reached, energy begins to be generated by the exothermic nuclear fusion process in which the core converts hydrogen into helium[9]。
Once the nuclear fusion of hydrogen becomes the main source of energy in the process of energy generation, gravity will have no excess energy to shrink the star[11], this star will follow a curve and fall on the standard main sequence belt called on the Herot diagram.Astronomers sometimes refer to this stage as "zero age main sequence belt", or ZAMS[12-13]。This curve is the point where the star begins nuclear fusion, and ZAMS can be calculated by computer model according to the characteristics of the star.From this point, the brightness and surface temperature of the star will increase with age.
Until a large amount of hydrogen in the core is consumed, the star is still near the initial position on the main sequence belt, and then it starts to become a brighter star (on the Herot chart, the evolution of the star is moving up and right away from the main sequence belt).Therefore, the main sequence zone is a stage of stellar life dominated by hydrogen combustion[9]。
Division of main sequence zone
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Main order bandyesHerotuOn the diagonal line curve, most stars are in this range, and the stars in this area are called main sequence stars ordwarf star, whereRed dwarfThe temperature of is the lowest.This line is very obvious, because only hydrogennuclear fusionThe stellar spectral type and brightness are directly related to the stellar massfixed starAlmost all of his life is spent at this stage.However, even under ideal observation, the main sequence band will still be somewhat ambiguous.For example, adjacentcompanion, rotation or magnetic field will cause some changes.Specifically, some stars with poor metal（subdwarf ）The position is just below the main sequence band. Nuclear fusion of hydrogen is also carried out, but the chemical composition of the lower end of the main sequence band will cause confusion.
Astronomers sometimes refer to the "zero age main sequence zone" (ZAMS), which is a curve obtained by calculation. It shows that when a star starts nuclear fusion of hydrogen, its brightness andsurface temperature Typical stars begin at this point with age, and their surface temperature and brightness increase.When a star is born, it will enter the main sequence belt, and leave the main sequence belt before dying.The sun is a main sequence star, 4.6 billion years old,Spectral classificationIt is G2V.When the hydrogen in the core is exhausted, it will expand into a red giant star.
The main sequence band is divided into upper and lower segments according to the main process of star energy generation.A series of main processes for stars with mass less than 1.5 times the mass of the sun to gather and fuse hydrogen into helium are called proton proton chain reaction.Beyond this mass in the upper part of the main sequence band, nuclear fusion is mainly carbon, nitrogen, and oxygen.Generally, the higher the mass, the shorter the lifetime of the star in the main sequence belt.After the nuclear fuel in the core has been exhausted, the development of stars will leave the main sequence belt on the Herot chart.At this time, the development of the star is determined by its mass. Stars with mass less than 0.23 solar mass will directly become white dwarfs, while stars with mass less than 10 solar mass will go through the stage of red giant stars;Stars with higher mass can explode into supernovae or collapse directly into black holes.
Related parameters
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If the star is regarded as an ideal energy radiator, that is, a black body, then the luminosity L, radius R and effective temperature
Can be usedStefan Boltzmann lawL=4 πσ RTeff.Here σ is Stefan-Boltzmann constant。If the star's position on the Herot chart shows its approximate brightness, this relationship can be used to estimate its radius.
The mass, radius, and brightness of stars are closely related, and their respective values can be estimated approximately.The first is Stefan Boltzmann law, which shows the relationship between brightness L, radius R and effective surface temperature Teff.Secondly, the relationship between brightness L and quality M is given.Finally, there is a nearly linear relationship between the mass M and the radius R.The factor relationship of M relative to R is between 2.5 and 3M.This relationship is approximately proportional to the temperature inside the star
, and its extremely slow increase reflects that the efficiency of energy generation in the core depends on the temperature, which is consistent with the mass light relationship.Therefore, too high or too low temperature will lead to instability of stars.
A good approximation is
, energy generation rate per unit mass, if ε is proportional to
, here
Is the core temperature.This at least applies to stars like the sun, showing thatCarbon nitrogen oxygen cycleR ∝ M is more suitable for stars of.
The following table shows the typical values of stars in the main sequence belt:luminosity(50) , radius (R), and mass (M) are both relative to the comparison values of the sun, a dwarf star classified as G2V by spectrum, and the correct values can have 20-30% changes.
(Note: the following data is not completely consistent with the external link, and the brightness per unit area does not follow the ratio of temperature (T))
All the main sequence stars are in progressnuclear fusionThe core area that generates energy.The temperature and density of the core must be able to maintain the production of polar energy and support the rest of the star.The reduction of energy generated will lead to the compression of the core by the mass covered outside, which will result in the increase of the rate of nuclear fusion due to higher temperature and pressure.Similarly, increasing energy production will cause the star to expand, reducing the pressure on the core.Therefore, stars form autonomously.All main sequence stars have core regions for nuclear fusion to generate energy.
The temperature and density of the core must be able to maintain the production of polar energy and support the rest of the star.The reduction of energy generated will lead to the compression of the core by the mass covered outside, which will result in the increase of the rate of nuclear fusion due to higher temperature and pressure.Similarly, increasing energy production will cause the star to expand, reducing the pressure on the core.Therefore, star formation is autonomousHydrostatic balanceSystem, so that the process is stable during the life of the main sequence band[14]。
The main sequence star has two types of hydrogen reaction processes, and the rate of energy generation of each type depends on the temperature of the core region.Astronomers divide the main sequence band into upper and lower parts according to which of the two types is the dominant process of nuclear fusion.In the lower part of the main sequence band, energy is mainly generated through proton proton chain reaction, and hydrogen is directly fused into helium through a series of steps.The stars in the upper part of the main sequence belt have enough high core temperature to be used effectivelyCarbon nitrogen oxygen cycle。This process uses carbon, nitrogen, and oxygen atoms asCatalyst, in which hydrogen is fused into helium[15]。
When the temperature is 18 million K, the PP process and CNO cycle are equally effective, and each produces half of the star's net luminosity.The stellar mass of the core at this temperature is about 1.5 solar mass, and the value of stars in the upper part of the main sequence belt exceeds this value.Therefore, roughly speaking, stars with spectral type F or lower temperature are in the lower part of the main sequence band, and stars with spectral type A or hotter are in the upper part of the main sequence band.The transition from one major energy generation type to another has a mass range of less than one solar mass.In our sun, only 1.5% of the energy of stars with 1 solar mass is generated by CNO cycle[16]。In contrast, for stars with 1.8 solar mass or higher, almost all energy is output completely through CNO cycle[17]。
The upper limit of the observed star mass in the upper part of the main sequence star is 120 to 200 solar mass[18]。The theoretical explanation of this limitation is that the star whose mass exceeds cannot rapidly radiate energy to maintain stability, so any additional mass will be ejected in a series of expansion and contraction until the star reaches the limit of stable state[19]。The lower mass limit for continuous proton proton chain reaction is about 0.08 solar mass, and sub stellar objects below this threshold cannot maintain hydrogen fusion, such as known brown dwarfs[20]。
Evolutionary trajectory
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Once the main sequence star consumes the hydrogen in its core, the resulting energy loss will lead to gravitational collapse.For stars with mass less than 0.23 solar mass, once the hydrogen in the core stops breeding energy, it is predicted that they will directly become white dwarfs.When the hydrogen around the helium core reaches enough temperature and pressure, the stars with a mass of 10 solar masses exceeding this critical value will start nuclear fusion and become hydrogen combustion shells.In addition to this change, the envelope outside the star will also expand and cause the temperature to drop, which will turn into a red giant star.At this time, the star terminates its evolution on the main sequence belt and enters the giant star branch.The path of star evolution crosses the Herot chart and moves to the upper right corner of the main sequence belt, which is called the evolution path.
The helium core of the red giant continues to collapse until it is completely affectedelectron degeneracy pressure - Onequantum mechanicsThe effect of limiting the compactness of the material that can be compressed - support.For stars with more than 0.5 solar mass[21]The core can reach high enough temperature to burn helium into carbon through the process of 3 helium[22-23]。Stars with masses between 5 and 7.5 solar masses may have produced elements with higher atomic weights by nuclear fusion.For stars with solar mass or heavier, this process will make the core more and more compact, eventually leading to the collapse of the core, throwing out the gas shell covering the outside of the star and causing the explosion of type II supernova, type Ib supernova, or type Ic supernova.
When aclusterWhen nearly all stars form at the same time, the lifetime of these stars will depend on the individual mass.The star with the largest mass will leave the main sequence band first, and then the star with the lower mass will leave in order.Therefore, the evolution of stars will leave from the most massive ones and turn to the right side of the Herot chart according to their positions in the main sequence belt.The current position of stars in this cluster leaving the main sequence belt is the so-called turning point, which can be used to estimate the age of the cluster[24]。
